1. Field of the Invention
This invention relates to magnetic bubble domain devices, and more particularly to magnetic bubble domain devices utilizing as a bubble domain material a rare earth iron garnet characterized by the same rare earth magnetic ion in all available dodecahedral lattice sites.
2. Description of the Prior Art
Magnetic bubble domain devices and systems are well known in the prior art and have conveniently used mixed rare earth iron garnet materials. For example U.S. Pat. No. 3,701,125 shows a complete magnetic bubble domain system for performing the functions of writing, storage, reading, where uniaxial magnetic iron garnets are mentioned as suitable bubble domain materials.
Rare earth iron garnets are part of a family of nominally cubic, ferrimagnetic compounds having the space group Oh(10)-Ia3d and 8 A.sub.3 Fe.sub.5 O.sub.12 formula units per unit cell (15). This cell accommodates 96 oxygens and has an edge approximately 12.5 Angstroms in length. There are 24 tetrahedral and 16 octahedral sites within the oxygen array that accommodate Fe and cations of comparable size. There are also 24 dodecahedral sites that accommodate larger cations, such as Y, La, rare earth ions, Bi and Ca ions.
The magnetic moments of the cations in the tetrahedral sites are aligned together. The magnetic moments of the cations in the octahedral sites are also aligned together but are in opposition to those in the tetrahedral sites. Consequently, a net magnetic moment can be established if there are different numbers of Fe ions in the tetrahedral and octahedral sites. This magnetic moment can be decreased by substituting for some of the Fe in the tetrahedral sites, using cations such as Al.sup.3.sup.+, Ga.sup.3.sup.+, Si.sup.4.sup.+, or V.sup.5.sup.+. The magnetic moment can be affected in the opposite way by substitution for the Fe ions in the octahedral sites, using for example cations such as Zn.sup.2.sup.+ or Se.sup.3.sup.+.
There has been much discussion of anisotropy in these rare earth iron garnets in the literature. For instance, a review of growth induced magnetic anisotropy is contained in A. Rosencwaig et al, AIP Conference Proceedings No. 5, Magnetism and Magnetic Materials-1971 (American Institute of Physics, New York, 1972, page 57). In the prior art, growth induced anisotropy is explained in terms of a site preference or pair ordering model in which two or more rare earth ions or one or more rare earth ions and another c-site ion such as Y, are required. A non-random occupation of the c-sites during growth causes an induced uniaxial anisotropy. That is, non cubic anisotropy arises from the differential occupation by different rear earth ions of sites which are inequivalent during the growth process, while being crystallographically equivalent.
U.S. Pat. No. 3,646,529 describes rare earth iron garnets which are suitable for magnetic bubble domain devices at room temperature ranges. These garnet materials have growth induced anisotropy and require a substitution for Fe in order to reduce the magnetic moment due to iron in tetrahedral sites of the material. In addition, preferred compositions use at least two ions in the dodecahedral sites, at least one of which is a moderately weak magnetic iron such as Gd, Tb, Dy, Ho, Eu, and Tm. A non-weak or oppositely aligned magnetic ion such as Sm, Y, La, Ce, Pr, Nd, Yb and Lu is stated as being insufficient by itself to produce favorable bubble domain materials. These garnets are grown at temperatures up to approximately 1200.degree.C in order to obtain the necessary anisotropy.
In this reference, it is stated that garnets containing Gd, Tb, and Dy have magnetizations which are strongly temperature dependent. Compositions using 100 percent of any one of these ions in the dodecahedral lattice sites, without an Fe substitution, are generally undesirable for room temperature device operation, due to their very high magnetization and due to an adverse temperature dependence. Consequently, this reference does not suggest using single rare earth ions in all the dodecahedral lattice sites without an Fe substitution, and in particular does not suggest 100 percent Eu or Sm garnets, with or without Fe substitutions.
U.S. Pat. No. 3,665,427 describes various magnetic bubble domain devices utilizing garnet compositions. These compositions contain at least two ions in the dodecahedral sites which are selected in a particular way so as to reduce magnetostriction in the &lt;111&gt; direction and also in the &lt;100&gt; direction. In the optimum case, it is desired to reduce magnetostriction to a value equal to or very close to zero. Consequently, the dodecahedral sites are occupied by at least two ions of different magnetostriction sign. Preparation of these garnet materials involves growth at temperatures below approximately 1200.degree.C to insure ordering which is needed for magnetically uniaxial alignment. A rare earth iron garnet with only one rare earth ion occupying the available dodecahedral sites is not suitable as a bubble domain material according to the teaching of this reference.
U.S. Pat. No. 3,645,788 also describes magnetic devices using magnetic iron garnet materials. In these materials, a stress induced uniaxial anisotropy results due to thermal expansion coefficient differences between the film and the substrate during cooling from the deposition temperature. That is, there must be sufficient mechanical strain in the garnet film to provide the film with a sufficient uniaxial anisotropy for the formation of magnetic bubble domains therein. In these rare earth iron garnet films, the dodecahedral sites are occupied by at least two ions.
Other theories have been proposed for non-cubic anisotropy in garnets which have no magnetic ions in the rare earth sites and in which Fe.sup.3.sup.+ fills all the octahedral and tetrahedral sites. Anisotropy has been sometimes proposed in such systems as the result of interactions between the Fe sublattices. Other theories such as the "dirt effect" have been proposed to explain this anisotropy. Another theory (W. T. Stacy et al, Solid State Communications 9, page 2005, 1971) depends on the fact that oxygen vacancies can be incorporated in substantial quantities during the growth process. These oxygen vacancies produce large uniaxial crystal fields at the neighboring Fe.sup.3.sup.+ ions. However, this theory for growth induced anisotropy has been debated, as being inconsistent with data concerning the diffusion of oxygen in garnets.
Another theory of anisotropy is that due to Akselrad et al (Applied Physics Letters 19, 464, 1971) which depends upon the fact that the crystal field at a tetrahedral or octahedral site is affected by the size and nature of the ion on nearby dodecahedral sites. Ordering of the non-magnetic ions on the dodecahedral sites is suggested as leading to growth induced anisotropy through the effect of these ions on the crystal field at neighboring Fe.sup.3.sup.+ ions. However, this theory does not account well for effects in pure garnets such as Y.sub.3 Fe.sub.5 O.sub.12.
This prior art describing various iron garnet materials suitable for bubble domains leads away from the concept of a room temperature bubble domain device using a rare earth iron garnet material having the same rare earth ion on all of the available dodecahedral lattice sites (hereafter to be called a single rare earth iron garnet). That is, the prior art does not suggest that sufficient uniaxial anisotropy can be obtained in such a garnet in order to support stable magnetic bubble domains at room temperature ranges. The growth induced anisotropies actually obtained in the garnets of this invention cannot be explained as a result of pair ordering or site preference theories, nor can they be explained by an interaction between Fe sublattices. In fact, the prior art theories would lead one to believe that very negligible or zero growth induced uniaxial anisotropy would be obtained in such garnet materials and that the magnetic moment of the material would be so high as to preclude any stable magnetic bubble domain formation. However, it has been discovered that magnetic bubble domain materials containing the same rare earth ion on all available dodecahedral sites, with and without Fe substitutions, can be used to provide stable room temperature magnetic bubble domain devices. Additionally, at least one of the rare earth ions that can be used is one which the prior art would term too weakly magnetic to be used alone with Fe in a garnet for provision of suitable magnetic bubble domain devices.
In addition to the above, an unexpected temperature dependence of the induced uniaxial anisotropy constant K.sub.u was found in these inventive garnet bubble domain devices. This anisotropy increased dramatically when the deposition temperature was lowered and did not approach a zero magnitude as compositions approaching a single rare earth iron garnet were reached.
Numerous advantages result when these single rare earth iron garnet materials are used in room temperature bubble domain devices. For instance, the minimum number of components in these garnets leads to increased process reliability and economy. In addition, processing becomes more simple and involves only a minimum number of components. Also, with certain of these films a garnet substrate can be provided using a Pt crucible for growth rather than an Ir crucible. This reduces fabrication cost even more. Another advantage is that the films can be grown with various thicknesses and will support small magnetic bubble domains of about 1 micron in diameter.
Accordingly, it is a primary object of this invention to provide simple garnet compositions which can be used in bubble domain devices which have stable bubble domains during operation at room temperature ranges.
It is another object of this invention to provide room temperature magnetic bubble domain devices using magnetic materials which are easy to fabricate and utilize only a minimum number of components.
It is still another object of this invention to provide magnetic bubble domain devices using magnetic materials which are economical to fabricate.
It is still another object of this invention to provide magnetic bubble domain devices using magnetic materials which are capable of supporting very small magnetic bubble domains which are stable over room temperature ranges.
It is a further object of this invention to provide magnetic bubble domain devices using improved garnet materials which are very simple and are economical to fabricate.
It is a still further object of this invention to provide room temperature bubble domain devices using improved garnet materials having different crystallographic orientations.